Sputtering is a method of depositing a film on a substrate by accelerating ions typically by means of glow discharge so as to hit a sputtering target so that the target ejects a material for the film due to the kinetic energy of the ions. It is widely used as a tool for fabricating structures such as thin film devices in the areas of semiconductors, liquid crystals, solar cells, and so on. Among others, a target using an alumina sintered body (hereinafter, also referred to as an “alumina target”) is used as a source of film materials in forming a protective film or an insulating film in magnetic heads, magneto-optical disks and the like. It is also used for forming a wear resistant film on cemented carbide tools.
Since an alumina target is usually an insulating material, a high-frequency sputtering apparatus is used for forming an alumina sputtered film. In this apparatus, an alumina target is joined to an electrode, a substrate is disposed at a counter electrode of the electrode, and a sputtering discharge is carried out in an atmosphere such as argon under a reduced pressure, thereby depositing an alumina film on the substrate.
Patent Document 1 discloses an alumina sputtering target which uses an alumina sintered body having an average grain size of not less than 5 μm and not more than 20 μm, and a porosity of not less than 0.3% and not more than 1.5%. Particularly, it is described in claim 2 thereof that an alumina sintered body having a purity of not less than 99.9% (generally, called as “three nine” and denoted as “3N”) is used, and is also shown in an embodiment that an example using alumina powder of 3N is used.